Thin film resistors are widely used in the integrated circuit and they are generally fabricated during back end of the line (BEOL) processing. They offer resistances that are better defined than corresponding resistors formed by diffusion into the semiconductor substrate. Additionally, resistors formed in the back end will have less parasitic capacitive coupling to the semiconductor substrate than resistors formed in the front end.
In the past, the integration of the process steps to form precision BEOL resistor was straightforward, as the distance between metal levels in the back end provided sufficient room.
As technology advances and the dimensions of structures shrink, the distance between levels in the back end decreases, while the thickness of the resistor films remains the same, so that the conventional back end resistor takes up a greater fraction of the inter-level spacing. The result is that it becomes increasingly difficult to fabricate precision BEOL thin film resistor between two metal wiring levels.
Accordingly, there remains a need for a novel fabrication technique for forming precision BEOL thin film resistor on the integrated circuit structures.
Additionally, the advancement of semiconductor technology has lead to the introduction of low-k materials such as FSG (fluorinated silica glass) and SiCOH (silicon oxycarbide) films into BEOL processing. The presence of Fluorine and Hydrogen species presents special challenges to the resistor film stability because diffusion of Fluorine and Hydrogen could lead to the poison of resistive films which could result in resistance shift.
In view of the foregoing, there is a need in the art for a novel fabrication method for forming thin film precision BEOL resistor and a technique to protect resistors from interacting with BEOL ILD dielectric material.